The present invention relates to a radio telephone and, more particularly, to a security module which protects a KEY code particular to a given radio telephone against illicit use.
Generally, a wired telephone has a channel fixed by a line and, is, therefore, used at a limited place and by a limited person or persons. On the other hand, a wireless or radio telephone is connected to a base station together with other numerous radio telephones by common radio communication media. It is likely, therefore, that an unauthorized person uses the radio telephone at an unexpected place. To eliminate such illicit use, it has been customary to assign a particular identification (ID) code to each radio telephone in order to determine whether or not a radio telephone of interest is authentic one. Specifically, a PROM storing a particular ID code is incorporated in each radio telephone, so that the ID code may be read out to see if the telephone of interest is acceptable.
A modern radio telephone, especially portable radio telephone, has a miniature configuration and cannot readily be designed such that the casing thereof is openable for loading a PROM. In light of this, a current trend is toward the use of an E.sup.2 PROM (electrically rewritable non-volatile ROM) in place of a PROM. Specifically, after an E.sup.2 PROM has been built in a radio telephone on the manufacturer's production line, an ID code or similar information is written to the E.sup.2 PROM from the outside. The ID code is usually written to the E.sup.2 PROM via a keypad provided on the handset of the telephone. Stated another way, the ID code stored in the E.sup.2 PROM can be changed on the keypad of the handset, as desired. It is likely, therefore, that once the ID code particular to a given radio telephone is disclosed, an unauthorized person writes it in another radio telephone and uses this telephone illicitly. Another conventional approach for authentication is to add a passsword before writing an ID code or to allow an ID code to be written only when a particular terminal is lowered to the ground level by a test link. However, even the password or the test link level cannot fully protect a radio telephone against illicit use since it is easy to clear.
Attempts have recently been made to store a KEY code in a radio telephone in addition to an ID code, so that communication may be implemented by a signal encrypted by the KEY code. Specifically, a radio telephone encrypts random data sent from a base station by a KEY code and then returns the resulted data to the base station. In response, the base station determiners whether or not the encrypted data accurately matches the ID code and, only if the former matches the latter, sets up call connection. This kind of scheme is disclosed in "SIS ADDENDUM TO NMT DOC 900-1 and 900-3", July 1988. Even when an unauthorized person happens to know an ID code and encrypted data by monitoring the radio channel with some device, the ID code cannot be used unless the person knows the KEY code.
However, despite the above-stated implementation, the memory storing the KEY code is accessible to read out the key code. In this sese, the protection using the KEY code is not meaningful.